Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss. Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths, thereby increasing network capacity.
The data communicated in a optical signal (or a particular multiplexed channel thereof) may be modulated using a variety of different modulation techniques. Such techniques include, as examples, on-off keying (OOK) and return-to-zero (RZ) phase shift keying (PSK). Both of these modulation techniques produce single polarization (SP) signals. Other modulation techniques, such as dual polarization (DP) phase shift keying produce dual polarization signals. For example, dual polarization quadrature phase shift keying (DP-QPSK) modulation is being contemplated for use in modulating one hundred gigabit/second signals in next generation optical networks.
One issue that exists with DP signals is signal degradation due to cross-phase modulation (XPM). XPM causes the state of polarization of each symbol to rotate differently as the signal propagates in the optical fiber. This results in polarization scattering, which depolarizes the signal and causes crosstalk between the two polarization components of the signal. This effect is data dependent and occurs on a timescale of baud rate.